For many years, it has been a goal of the printing industry to form printing images directly from an electronically composed digital database, for example, by a so-called “computer-to-plate” system. The advantages of such a system over the traditional methods of making printing plates are the elimination of the costly intermediate silver-containing film and processing chemicals; a saving of time; and the ability to automate the system with consequent reduction in labor costs.
The introduction of laser technology provided the first opportunity to form an image directly on a printing plate precursor by directing a laser beam at sequential areas of the printing plate precursor and modulating the beam so as to vary its intensity. In this way, radiation sensitive plates comprising a high sensitivity photocrosslinkable polymer coating have been exposed to imagewise distributions of radiation from various laser sources and electrophotographic printing plate precursors having sensitivity ranging from the visible spectral region into the near infra-red region (including thermal sensitivity) have been successfully exposed using low powered air-cooled argon-ion lasers and semiconductor laser devices.
While lithographic printing precursors post-exposure developable using aqueous media, preferably alkaline aqueous media, are well known and widely used in the printing industry, there is a more specific subset of precursors that may be developed on press by the action of the fountain solution employed during wet offset printing. A newer class of lithographic media is based upon the general concept of employing polymeric particles in an otherwise hydrophilic binder, often along with a substance to convert light into heat. This kind of media is exemplified by U.S. Pat. No. 6,001,536. The unilluminated areas of a lithographic precursor based on this generic media may be removed by treatment with fountain solution on a printing press. This kind of precursor is therefore pseudo-processless, in that no specific separate development step with a specific developer, as such, is required to obtain a master. The illuminated areas are rendered hydrophobic and hence the master is in effect negative-working. These precursors allow lithographic printing masters to be made relatively easily on-press, but suffer from poor run length. The quality of the printed image rendered is directly dependent on the choice and quality of hydrophilic substrate used, as this substrate is exposed and has to carry the fountain solution during the wet offset printing process.
A more specific category of lithographic precursors employs mechanisms and compositions that cause the sensitive layer on the substrate to switch between hydrophilic and hydrophobic, without any material being required to be removed with a development step. That is, there is no removal of material at all, even by fountain solution. These are true processless precursors.
By way of example, U.S. Pat. No. 6,410,202 describes a composition for thermal imaging comprising a hydrophilic heat-sensitive polymer having recurring ionic groups within the polymer backbone or chemically attached thereto. The imaging members of this particular invention do not require post-imaging wet processing and are generally negative-working in nature. In some cases, the polymers are crosslinked upon exposure and provide increased durability to the imaging members. In other and preferred cases, the polymers are crosslinked upon application to a support and curing. A further example of this class of precursor is provided by U.S. Pat. No. 5,985,514. That patent describes an imaging member that is composed of a hydrophilic imaging layer having a hydrophilic heat-sensitive polymer containing heat-activatable thiosulfate groups, and optionally a photothermal conversion material. Upon application of energy that generates heat, such as from IR irradiation, the polymer is crosslinked and rendered more hydrophobic. The exposed imaging member can be contacted with a lithographic printing ink and a fountain solution and used for printing with or without post-imaging wet processing. U.S. Pat. No. 4,081,572 describes making hydrophilic printing masters comprising coating a self-supporting master substrate with a specific hydrophilic polymer containing carboxylic acid functionality and selectively converting this polymer in image configuration to a hydrophobic condition by heat. The polymer is selectively converted to a hydrophobic condition in image configuration through heat-induced cyclodehydration reactions. In other examples the precursor is inherently positive-working, as in the case of U.S. Pat. No. 4,634,659. That particular patent describes a method of making a processing-free planographic printing plate comprising irradiating a plate surface comprised of a hydrophobic organic compound capable of being converted, upon exposure to radiation, from hydrophobic to hydrophilic, carrying out the exposure in an image pattern, thereby selectively converting said surface, in the image pattern, from hydrophobic to hydrophilic, thereby making th precursor positive-working.
A yet more specific category of true processless lithographic precursors, is based on media comprising polymer-based particles or microcapsules:
In U.S. Pat. No. 6,550,237 a heat-sensitive material is described for making a negative working non-ablative lithographic printing plate including in a heat sensitive layer thermoplastic polymer beads and a compound capable of converting light into heat on a surface of a hydrophilic metal support. The layer is free of binder, and is characterized in that the thermoplastic polymer beads have a diameter between 0.2 μm and 1.4 μm. Argument is provided for the requirement that the thermoplastic particles should have a specific size range. It is explained that, when the polymer particles are subjected to a temperature above the coagulation temperature they coagulate to form a hydrophobic agglomerate so that at these parts the metallic support becomes hydrophobic and oleophilic. Preferably, the polymer particles are selected from the group consisting of polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, polyvinyl carbazole etc., copolymers or mixtures thereof. Most preferably used are polystyrene, polyacrylate or copolymers thereof and polyesters or phenolic resins. No indication is given that the polymer particles should be hydrophilic, or that there may be more than one polymer in the particles.
In European Patent Application No. EP01057622, a lithographic printing plate precursor requiring no development step is described. It comprises a support, having provided thereon a layer comprising a hydrophilic medium, wherein the layer comprising a hydrophilic medium contains a hydrophobitization precursor having a hydrophilic surface and a light/heat converting agent which is hydrophilic in itself, or at least on the surface. Various implementations of the invention are presented in which the hydrophobitization precursor having a hydrophilic surface is a particle dispersion of composite constitution containing a hydrophobic substance at the core part and having a surface layer of specifically superficial hydrophilicity. All forms of particles diclosed are composed of either one or two distinct materials. Various materials may be at the core, including hydrophobic polymeric materials and crosslinking materials. A light-to-heat converting material, which is specifically chosen to be hydrophilic, is also added.
U.S. Pat. No. 5,569,573 describes a thermosensitive lithographic printing original plate comprising a substrate, a hydrophilic layer containing a hydrophilic binder polymer, and a microcapsuled oleophilic material which forms an image area by heating; the hydrophilic binder polymer having a three-dimensional cross-link and a functional group which chemically combines with the oleophilic material in the microcapsule when the microcapsule is ruptured, and the microcapsuled oleophilic material having a functional group which chemically combines with the hydrophilic binder polymer when the microcapsule is ruptured. Among the many hydrophilic binder polymers listed are polysaccharides.
There remains a requirement for negative-working, true processless, lithographic precursors having long run-length, suitable sensitivity to laser-diode-based imaging radiation, and which are easy to prepare, preferably from aqueous media.